RFR: 8340114: Remove outdated SelectVersion() function from the launcher and update the code comments explaining the code flow

[Jaikiran Pai]

Can I please get a review of this change which proposes to remove the (internal) `SelectVersion()` function from the java launcher and also update the code comments in the launcher to match the current implementation?

As noted in https://bugs.openjdk.org/browse/JDK-8340114, the `SelectVersion()` function in the launcher used to be relevant when JRE selection was a feature. That feature has been removed since Java 9 https://bugs.openjdk.org/browse/JDK-8058407. The SelectVersion() in its current form isn't relevant anymore and can be removed.

While at it, it was noticed that the current "flowchart" code comments in the launcher which attempts to explain the flow in the launcher code are outdated. The commit in this PR updates those comments for macosx and unix implementation. The windows variant doesn't have a "flowchart", but it too deserves a high level comment explaining this flow. I haven't updated the windows variant in this PR because that does a few additional things, which I need to review and understand better. I plan to take that up in a future change.

An existing `test/jdk/tools/launcher/MultipleJRERemoved.java/MultipleJRERemoved` test had to be updated due to the changes in this PR. That test was launching `java` (once) with 3 unsupported JRE selection options and was expecting 3 error messages (one each for the unsupported option) for that single launch. With the change in this PR, we don't accumulate and throw all those 3 errors and instead we fail fast for any of these 3 unsupported JRE selection options. The fail fast implementation matches what we do with other similar unsupported options. The test had to be updated to not expect all 3 errors message in a single launch and instead expect to find one of those error messages. Given what this test is for, and the fact that JRE version selection options (rightly) continue to raise an error after this change, I think, an update to that test should be OK.

No new tests have been introduced in this PR and tier testing is currently in progress.

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Commit messages:
 - 8340114: Remove outdated SelectVersion() function from the launcher and update the code comments explaining the code flow

Changes: https://git.openjdk.org/jdk/pull/20997/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20997&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340114
  Stats: 541 lines in 7 files changed: 99 ins; 306 del; 136 mod
  Patch: https://git.openjdk.org/jdk/pull/20997.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20997/head:pull/20997

PR: https://git.openjdk.org/jdk/pull/20997